The mystery called Havana syndrome becomes clearer when one question is split into three: what people experienced, what could cause it, and whether anyone deliberately caused it.

One name, many reports

U.S. personnel first reported unusual incidents in Havana in 2016. Some described sudden sounds or pressure followed by symptoms such as headache, dizziness, balance problems or cognitive difficulty. The government now uses the broader term anomalous health incidents, or AHIs. That wording matters: it groups reports for investigation without declaring that every case shares one disease or cause. [1][4]

Three questions, three kinds of evidence

The evidence sits in three different boxes. Medicine asks whether participants share a measurable pattern of injury. Physics asks whether a proposed mechanism could produce the reported sensations. Intelligence asks whether a foreign actor possessed and used such a mechanism. An answer in one box does not automatically answer the others. A plausible device is not evidence that it was present at a specific location; genuine symptoms do not reveal who or what caused them. [1][2][5]

What medicine found

In 2024, NIH researchers compared 81 people who reported AHIs with 48 matched controls using structural, diffusion and functional MRI. Scans began as early as 14 days after an incident, although the median delay was 80 days. After correcting for multiple comparisons, the study found no significant group differences in any MRI modality. A companion clinical investigation likewise found no significant differences in most tested measures. The researchers still stressed that participants’ symptoms were real and sometimes disabling. They also noted that a transient event could leave no persistent MRI signal, or one that current methods and sample size could not detect. [2][3][4]

Why radiofrequency energy stayed in the discussion

Radiofrequency energy remains part of the discussion because a 2020 National Academies committee found that several sudden, directional features reported in the original Havana cases were compatible with pulsed RF effects. Compatibility is not detected exposure. The committee lacked measurements of a source, dose, frequency and timing at the incident sites, and said those gaps prevented it from linking a specific biophysical effect to the cases. [5] A later intelligence expert panel judged pulsed electromagnetic energy plausible for a subset of reports, while also identifying information gaps and considering ultrasound plausible only in close-access situations. [6]

What intelligence agencies judged

The latest public Intelligence Community assessment, released in January 2025, did not reach one unanimous verdict. Five components judged foreign responsibility very unlikely, one judged it unlikely and one abstained. Years of collection had not produced a compelling link between a foreign actor and any specific reported event. Yet two components shifted their views about possible technology: one assigned approximately even odds that a foreign actor used a novel device against a small, undetermined subset; another assigned approximately even odds that such a capability existed but still judged deployment unlikely. Both shifts carried low confidence. [1]

What to keep in mind

The public record does not expose all evidence considered by intelligence agencies, and the NIH sample cannot represent every report. Delayed testing may miss short-lived changes. The available record supports uncertainty about a small subset, not a claim that all incidents share one cause—or that a secret weapon has been proved.

Sources & context

Numbered references link the explanation to its sources. Hypothetical examples and visual metaphors are labelled in the text. Checked 29 October 2026.

  1. Office of the Director of National Intelligence — Updated Assessment of Anomalous Health Incidents, as of December 2024 (released 10 January 2025)
  2. Pierpaoli et al. — Neuroimaging Findings in US Government Personnel and Their Family Members Involved in Anomalous Health Incidents, JAMA (18 March 2024)
  3. Chan et al. — Clinical, Biomarker, and Research Tests Among US Government Personnel and Their Family Members Involved in Anomalous Health Incidents, JAMA (18 March 2024)
  4. National Institutes of Health — NIH studies find severe symptoms but no MRI-detectable brain injury or biological abnormalities (18 March 2024)
  5. National Academies — An Assessment of Illness in U.S. Government Employees and Their Families at Overseas Embassies, Section 4 (2020)
  6. Office of the Director of National Intelligence — Complementary Efforts on Anomalous Health Incidents, expert panel findings (2 February 2022)
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